1. Field of the Invention
This invention is concerned with microporous, asymmetric polyimide films and their use as semipermeable membranes.
2. Description of the Prior Art
The publications listed below show the preparation of polyamic acids from diamines and tetracarboxylic acid dianhydrides, and the conversion of these polyamic acids to polyimides by treatment with heat alone or with chemical dehydrating agents.
U.s. pat. No. 3,179,630 (A. L. Endrey, 1965) PA1 U.s. pat. No. 3,179,632 (W. R. Hendrix, 1965) PA1 U.s. pat. No. 3,424,718 (R. J. Angelo, 1969) PA1 H. lee, D. Stoffey & K. Neville, "New Linear Polymers", McGraw-Hill, Inc. pp. 205-264 (1967) PA1 W. r. sorenson & T. W. Campbell, "Preparative Methods of Polymer Chemistry", 2nd Ed., Interscience Publishers, pp. 170-1 (1968) PA1 G. b. vaughan, J. C. Rose & G. P. Brown, "Polymer Preprints", 11, 339-46 (1970) PA1 H. scott, F. L. Serafin, P. L. Kronick, Polymer Letters 8, 563-71 (1970).
The process involves spreading a viscous solution of a polyamic acid in an organic solvent onto a solid surface and immersing the assembly in an organic solution of a dehydrating agent such as a combination of acetic anhydride and pyridine whereby a polyimide film is obtained. These films are symmetrical, i.e., of uniform density, since their cross-sections appear uniform throughout. Such uniformly dense films are of limited value as semipermeable membranes.
British patent specification No. 1,212,758 and South African patent application No. 68/5860, filed Sept. 3, 1968, disclose the concept of preparing asymmetric membranes from a variety of polymers, including polyimides. The disclosures show dissolving an already preformed polymer in an organic solvent to form a viscous solution, forming a polymeric film from such solution and then leaching out the solvent to produce an asymmetric membrane. Such membranes, being soluble in organic solvents, also have limited utility.